1. Field of the invention
The present invention relates to solid oxide fuel cells, a process for producing solid electrolyte films and a process for producing solid oxide fuel cells using such solid electrolyte films.
2. Description of the Related Art
Since solid oxide fuel cells (SOFC) operate at high temperatures of 1000.degree. C. or more, the activity of the electrodes of the SOFC is extremely high. Thus, an expensive catalyst of a noble metal such as platinum is not necessary. In addition, since the SOFC has a low polarization and a relatively high output voltage, its energy conversion efficiency is conspicuously higher than that of other fuel cells. Furthermore, since all the constituent materials of the SOFC are solid, the SOFC is stable and has a long use life. Recently, a stabilized zirconia and a composite oxide, having a perovskite structure and including lanthanum, are the most common and promising materials for the constituent materials of a solid electrolyte film and of an air electrode of the SOFC, respectively (`A Comprehensive Engineering For Energies` 13-2, 1990).
Manufacturing processes of solid electrolyte films and air electrodes are generally categorized as a dry process and a wet process. An EVD method and a spray method are most common as the dry process. The wet process includes a tape casting method, a slip casting method, an extrusion method and the like (`A Comprehensive Engineering For Energies` 13-2, 1990).
When the solid electrolyte films and the air electrodes are manufactured by a vapor phase method such as a chemical vapor deposition (CVD), an electrochemically vapor deposition (EVD) or the like, an apparatus with a large scale is needed, and the treated area and the treating speed are too small. Moreover, in the vapor phase method, zirconium chloride or the like is used and aqueous vapor mixed with oxygen is used, therefore a running cost of the apparatus applying the vapor phase method is high.
When the solid electrolyte films are formed by applying a plasma spraying method, the film-forming speed is great, an apparatus applying the plasma spraying method can be handled easily and the films can be formed as thin and relatively dense films. For these merits, the plasma spraying method has heretofore been used for forming the solid electrolyte films (`Sunshine` 1981, Vol. 2, No. 1: `A Comprehensive Engineering For Energies` 13-2, 1990).
The plasma spraying method is suitable for mass production, however, it is difficult to obtain an air tight film. On the other hand, the solid electrolyte film of the SOFC must be air tight to prevent lowering of the output due to leakage of fuel through the film. When applying the plasma spraying method, it was necessary to produce the solid electrolyte film with a thickness of not less than 200-300 .mu.m for satisfying the above air-tight performance required for the solid electrolyte film of the SOFC (Japanese Patent Application Opened Number 128,566/1988). However, an electrical resistance of such a solid electrolyte film composed of zirconia is relatively high and contributes to the internal resistance of the single cell as the main factor. It is necessary to solve such problems. Therefore, it is required to produce a SOFC single cell, in which its solid electrolyte film is produced by applying a spraying method, having an opened circuit voltage near its theoretical value and to reduce the internal resistance of the single cell.
Besides, a porosity of the solid electrolyte film formed by the plasma spraying method is generally more than 5% and reaches up to 10%, so that the plasma-sprayed solid electrolyte film does not have sufficient density for a solid electrolyte film of a SOFC. That is, cracks or laminar defects occur in the film during the plasma spraying. For this reason, leakage of a fuel occurs in that the fuel, such as hydrogen or carbon monoxide, penetrates the film when operating the SOFC. Consequently, an electromotive force per a single cell of the SOFC is frequently smaller than that of an ideal SOFC in which the leakage does not occur, so that the output of the SOFC and a conversion rate of the fuel to electric power are reduced. Therefore, it is desired to improve the density of the solid electrolyte portion of the solid oxide fuel cell for preventing the leakage of the fuel when thinning the solid electrolyte portion so that the output per the single cell of the SOFC may be improved.
Besides, in the SOFC, a thinner film is preferable as the solid electrolyte film. However, when a vapor phase method, such as a chemical vapor deposition (CVD), an electrochemically vapor deposition (EVD) or the like, is applied for thinning the electrolyte film, an apparatus with a large scale is needed, and the treated area and the treating speed are too small. Therefore, a cost for producing the film is high, it is difficult to produce the solid electrolyte film with a large area, and moreover, the above EVD method may be applicable only when the solid electrolyte film is formed onto a tubular substrate.
Further, it is known that a grain size of a raw material to be plasma sprayed, in which cerium oxide or zirconium oxide is solid solved with an oxide of an alkaline earth metal element or a rare earth element, is adjusted, and then the raw material is plasma sprayed to form a solid electrolyte film (Japanese Patent Application Opened Numbers 198,569/1986 and 198,570/1986).
However, a film produced by the plasma spraying is not generally densified enough. Therefore, when a solid electrolyte film of a SOFC is produced by the plasma spraying as described above, such film does not have a sufficient density required for the SOFC. For this reason, a leakage of a fuel occurs in that the fuel, such as hydrogen or carbon monoxide, penetrates the film when operating the SOFC. Consequently, an electromotive force per a single cell of the SOFC is smaller than, for example, 1 V in an ideal SOFC in which the leakage does not occur, so that an output of the SOFC and a conversion rate of the fuel to electric power are reduced.
In this case, it may be considered that a thickness of the solid electrolyte film is increased to prevent the leakage of the fuel. However, a resistance against diffusion of ions in the film is increased in this case, and consequently, a resistance of the cell becomes large. Therefore, a technique is desired for improving the output of power generation per the single cell by densifying the solid electrolyte film and thinning the same without causing the leakage of the fuel.
As a method for solving such problems, in Japanese Patent Application Opened Numbers 115,469/1992 and 62,459/1991, a method is disclosed that a solid electrolyte film of a SOFC is formed by plasma spraying and the film thus formed by plasma spraying is then subjected to a heat treatment. However, the film thus formed by plasma spraying contains many defects therein, so that it is difficult to obtain a film having a sufficient air-tight performance. In particular, for obtaining a high quality film having a permeation coefficient of N.sub.2 gas not more than 10.sup.-7 cm.sup.4 g.sup.-1 s.sup.-1, it is necessary to subject the film formed by plasma spraying to the heat treatment for a long time at a high temperature not lower than 1550.degree. C.
Much energy and time are consumed for carrying out the above heat treatment for a long time at such a high temperature, so that a cost for producing the film is considerably increased. Moreover, the film produced by plasma spraying adheres to a substrate which is also subjected to the heat treatment at a high temperature not lower than 1550.degree. C., so that the substrate may be deteriorated or deformed during the heat treatment. In particular, a porous substrate is used in the SOFC. Such a porous substrate is sintered at a high temperature not lower than 1550.degree. C., so that a dimension of the porous substrate becomes small and permeation of an oxidant or a fuel in the porous substrate is interrupted. Consequently, the SOFC having such a porous substrate which has undergone the heat treatment does not work effectively.